The present invention relates to the fabrication of thin film transistor (TFT) liquid crystal displays and, more specifically, to a diffusion barrier multi-layer structure for a liquid crystal display of low-temperature poly Si (LTPS) thin film transistors.
With the advance of techniques for manufacturing thin-film transistors, the liquid crystal displays (LCD) are widely applied in various electronic products, such as calculators, personal digital assistants (PDAs), watches, laptops, digital cameras, and mobile phones, etc. due to their advantages as smaller size, less weight, lower power consumption and no radiation. Furthermore, since manufacturers aggressively invest in research and development and employ large-scale fabricating equipment, the decreasing cost of production makes the TFT-LCD devices more popular.
The TFT panels on the current market are mostly traditional amorphous silicon (a-Si) TFT LCDs. Since the LTPS thin film transistor LCDs are superior in resolution, brightness, size and anti-electromagnetic interferences, the LCD manufacturers are gradually focusing on such a technical field. Considering quality of films and requirements of mass production, an excimer laser annealing is employed in the LTPS process. The excimer laser is used as a heat source and the laser light through the projection system produces laser beams with uniform energy distribution projected onto the a-Si structure on the glass substrate. After the a-Si structure on the glass substrate absorbs the energy of the excimer laser, the a-Si structure is transformed to be a poly Si structure. The entire annealing process is performed below 600xc2x0 C. and a typical glass substrate can be used.
When the a-Si is transformed by laser irradiation into the poly-Si in the LTPS process, not only the silicon film is heated, but also the surface of the glass substrate under the silicon film rises in temperature by absorbing the energy of the silicon film. At this moment, the impurities in the glass substrate would diffuse into the silicon film due to high temperature and then the characteristics of the silicon film would be ruined to lose its desired semiconductor nature. To solve this problem, a buffer layer as a diffusion barrier layer is deposited between the silicon film and the glass substrate to obstruct the diffusion of the impurities.
In the current technology, the diffusion barrier layer is fabricated by increasing the density of the diffusion barrier film itself and hence, the diffusion coefficient is decreased to enhance the effect on preventing the impurities from diffusing. However, the increase in the density of the film results in a side effect of stress increase. Moreover, the thickness of the diffusion barrier film is added to alternatively enhance the effect on stopping the diffusion of the impurities, but the throughput is usually reduced in this way.
Therefore, the improvement in the fabrication of the diffusion barrier layer in the current LTPS process is urgently desired such that the diffusion barrier layer can effectively stop the impurities from diffusing.
The objective of the present invention is to provide an improvement in the structure and fabrication of multiple diffusion barrier films for a TFT LCD by the LTPS technology. By magnifying the discontinuous construction between two layers of the multiple diffusion barrier films, it can enhance the effect of the multiple diffusion barrier films on stopping the impurities from diffusing.
In a first embodiment of the present invention, a diffusion barrier multi-layer structure for LTPS thin film transistors and the process for fabricating thereof are disclosed. The diffusion barrier multi-layer structure is formed between an insulating substrate and a polysilicon film of the thin film transistor and includes a first diffusion barrier layer and a second diffusion barrier layer. The polysilicon film is above the insulating substrate. The first diffusion barrier layer is formed on the surface of the insulating substrate with a plasma treatment to increase the coarseness of the first diffusion barrier layer. The second diffusion barrier layer is formed on the surface of the first diffusion barrier layer.
Furthermore, another diffusion barrier multi-layer structure for LTPS thin film transistors and the process for fabricating thereof are disclosed in a second embodiment of the present invention. The diffusion barrier multi-layer structure is formed between an insulating substrate and a polysilicon film of the thin film transistor and includes a first diffusion barrier layer, a first impurity collecting layer and a second diffusion barrier layer. The first diffusion barrier layer is formed on the surface of the insulating substrate. The first impurity collecting layer is formed on the surface of the first diffusion barrier layer and is a porous structure. The second diffusion barrier layer is formed on the surface of the first impurity collecting layer.